Self Assessment for First-Time Filers: The Complete Guide

Filing a Self Assessment return for the first time can feel like being handed a form full of terms nobody explained to you first — UTR, payments on account, cash basis. None of it is actually complicated once it's unpacked in order, which is exactly what this guide does, from working out whether you need to register through to what happens after you've filed. For deadlines, typical costs and choosing a professional, our complete tax accountant guide covers that ground — or use our Self Assessment tax calculator to get a rough idea of what you might owe before you go any further.

Do You Actually Need to Register?

Not everyone needs to file a Self Assessment return, and it's worth confirming your position before assuming either way.

If more than one of these applies, or you're simply not sure, it's far safer to register than to assume you don't need to — HMRC can charge penalties for failing to notify them you should have registered, even before a return is late.

How to Register With HMRC

Registering is a one-time process (you don't re-register every year), and it's what generates the reference numbers you'll need to actually file.

• Register online at gov.uk, choosing the option that matches your situation (self-employed, not self-employed, or a partner in a partnership).
• HMRC will post you a Unique Taxpayer Reference (UTR) — a 10-digit number that identifies your Self Assessment record — usually within about 10 working days, longer if you're abroad.
• You'll also need to set up a Government Gateway account, if you don't already have one, to file online.
• Once your UTR arrives, you can also enrol for the Self Assessment online service specifically, which sometimes involves a separate activation code sent by post.
• Keep your UTR somewhere safe — you'll need it every year, and retrieving a lost one takes time you may not have close to the deadline.

The registration deadline is 5 October following the end of the tax year in which you need to start filing — for example, if you became self-employed at any point in the 2026/27 tax year (6 April 2026 to 5 April 2027), you'd need to register by 5 October 2027. Leaving registration until close to this deadline is common, but it doesn't leave much room if your UTR takes longer than expected to arrive, especially with the 31 January filing deadline not far behind it.

What Actually Counts as Income

One of the most common first-time questions is simply: what am I supposed to be declaring? The answer is broader than most people expect.

• Self-employment profit — income from freelancing, contracting, selling goods or services, or running a business as a sole trader.
• Rental income from any UK or overseas property, above the £1,000 property allowance.
• Dividend income above the dividend allowance, including from a company you own.
• Savings interest above your Personal Savings Allowance.
• Foreign income, including overseas pensions and investments.
• Income from things like tips, commission, or one-off freelance work, even if it feels informal.
• Gains from selling certain assets, such as shares, a second property, or in some cases cryptoassets — reported through the Capital Gains section of the return, sometimes with a separate, tighter reporting deadline.

A common assumption is that only your “main” income counts, with smaller or occasional income left out. HMRC's expectation is closer to the opposite — that all relevant income sources are included, however minor some of them might feel. If you're not sure whether something specific counts, that's a normal, sensible question to bring to a professional rather than guessing.

Allowable Expenses for the Self-Employed

If you're self-employed, you're taxed on your profit — income minus allowable business expenses — not your total income. Getting this right genuinely changes what you owe.

If your total self-employment or property income for the year is under £1,000, the trading allowance or property allowance can be used instead of claiming actual expenses — a simpler option worth knowing about if your figures are small and your expenses genuinely minimal, though you can't claim both the allowance and actual expenses on the same income.

Cash Basis vs Traditional Accounting

Most sole traders now use cash basis accounting by default — recording income when it's actually received and expenses when they're actually paid, rather than when they were invoiced or billed. It's simpler for most small, straightforward businesses, since it broadly follows what's happened in your bank account.

Traditional (accruals) accounting instead recognises income and expenses when they're earned or incurred, regardless of when money changes hands, which can suit businesses with more complex timing — significant unpaid invoices at year end, or stock that needs valuing, for example. You can generally opt out of cash basis and use traditional accounting instead if it suits your business better, though it's worth understanding why before switching rather than assuming one is simply “more proper” than the other.

Filing Online vs on Paper

Online filing is the route the vast majority of people use, and for good reason: it has a later deadline (31 January, versus 31 October for paper), it calculates your bill for you as you go, and it lets you save progress and come back to it.

Paper returns are still accepted, mainly used by people who can't file online or specifically prefer not to, but the earlier deadline catches people out if they've assumed the 31 January date applies regardless of format. Unless you have a specific reason to file on paper, online is almost always the more forgiving option for a first return.

Filling in Your Return, Step by Step

The exact sections you see depend on your situation, but the general shape of an online return follows a consistent pattern.

Gathering your figures before you start — bank statements, invoices, receipts, your P60 or P45 if you've also been employed — makes the actual form-filling far quicker than trying to hunt down documents partway through. Doing this weeks before the deadline rather than the night before also leaves time to ask someone a question if something doesn't look right.

Payments on Account, Explained Simply

This is the part that catches almost every first-time filer off guard: your first Self Assessment bill is often bigger than the tax you'd expect to owe on the year itself.

If your tax bill is over £1,000, and less than 80% of your income was already taxed at source (for example, through PAYE), HMRC generally asks for “payments on account” — advance payments towards the following year's bill, split into two instalments: 31 January (alongside your balancing payment for the year just finished) and 31 July. In practice, this means your first January payment can be one and a half times your actual tax bill for the year — the bill itself, plus the first instalment towards next year.

• This isn't an extra tax, or a mistake — it's tax you'd owe anyway, simply collected earlier.
• If your income drops significantly the following year, you can apply to reduce your payments on account rather than overpaying and waiting for a refund.
• Budgeting for this from the start avoids the shock many first-time filers describe when their January bill is far larger than the profit they made that year would suggest.

What Happens After You File

Submitting your return isn't quite the end of the process — a few things happen afterwards worth knowing about.

• You'll get an on-screen confirmation and calculation once you submit, showing exactly how your bill was worked out.
• You can generally amend an online return for up to 12 months after the filing deadline if you spot a mistake or missed something.
• HMRC expects you to keep the records behind your return — invoices, receipts, bank statements — for a set number of years afterwards, generally at least 5 years after the 31 January deadline for that tax year, in case they ask to see them.
• If HMRC opens a check into your return, this doesn't necessarily mean something is wrong — checks happen for all sorts of reasons, and having organised records makes responding to one far more straightforward.

Common First-Time Mistakes We See

• Registering too close to the 5 October deadline, then finding the UTR hasn't arrived in time to comfortably file by 31 January.
• Assuming only your “main” source of income needs declaring, and leaving out smaller or occasional income.
• Not budgeting for payments on account, then being caught out by a January bill larger than expected.
• Mixing up turnover with profit, and not realising expenses reduce what's actually taxed.
• Leaving everything until the last few days, with no time left to deal with a missing document or an unexpected question.
• Not keeping records as they go, then trying to reconstruct a year's worth of income and expenses from memory close to the deadline.

Getting Help With Your First Return

None of the above needs to be worked out alone. A professional who deals with first-time filers regularly can register you, work out what counts as income and expenses for your specific situation, and either guide you through filing it yourself or file it on your behalf entirely.

Getting help earlier in the year — rather than in the final days before 31 January — tends to make the whole process considerably calmer, and leaves time to actually ask questions rather than just get the form submitted.

Frequently Asked Questions

A few more common first-timer questions — see our full FAQ page for more, or get in touch directly if yours isn't covered here.